I noticed that the dolphin I've been using is an older revision. I just installed dolphin 2.0 official release. Which folder has the savedata of gc and wii games? because I wanted to copy it over to the new dolphin.

You don't need to copy it, just extract the new version over the old one. The new files don't write over any user files.

To answer where the save file is, did you do a search? This gets asked once a week. The folder names aren't very descriptive, instead inside the dolphin window, right-click on the rom name and click "Wii save folder" and the contents of that is your save data.